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Piercing a Cloud of Doubt on Global Warming

NEW YORK (TheStreet) -- Knowledge of the clouds overhead is at the root of many uncertainties regarding the Earth's climate.

The role of water vapor and clouds is often touted as a major source of uncertainty that makes undependable the computer models climate scientists rely on to predict global warming. This claim usually comes from global warming deniers who would rain on the parade of the Intergovernmental Panel on Climate Change, the influential U.N. committee with the ear of policymakers around the world.

Most global warming denial amounts to little more than hot air. But there's a little more substance to this particular criticism. As noted in the National Climate Assessment report released by the Obama administration Tuesday, the role of clouds, in particular, injects a great deal of uncertainty in predictions of how much the planet will warm.

The main driver of global warming is carbon dioxide emissions. CO2 is a greenhouse gas that is always present in the atmosphere. In general, when there's more of it, the planet is warmer; when there's less it's cooler. Some of the longwave radiation that bounces off the planet's surface -- energy caused by the sun's heat -- is reflected back down to the planet by the CO2 in the atmosphere, an effect known as radiative forcing.

Last week, I talked to Princeton University Geosciences and International Affairs Professor Dr. Michael Oppenheimer, a coordinating lead author on the IPCC's Working Group II report. In that phone conversation, he pointed out that the amount of warming caused directly by CO2 would level off at around 1.2 degrees if there were no other factors involved -- at that point, the more CO2 pumped into the atmosphere, the less difference it would make to the surface temperature.

"The difficulty comes in," he said, "because a lot of things change in reaction to that warming -- and that's called 'feedback.'"

An increase in water vapor is one of the most important feedbacks. As the surface layer of the atmosphere warms, more water evaporates. Like CO2, water in the air absorbs energy from the sun and the increased energy from the surface and radiates it out again, some of it back down to the surface.

Along with other elements, the accompanying increase in water vapor helps compound that small temperature increase caused by CO2. Using a gauge commonly referred to as "climate sensitivity," a pioneering report in 1979 by a team led by Jule Charney estimated that, taking feedbacks into account, the Earth would warm 3 degrees Celsius, plus or minus 1.5 degrees, if the amount of CO2 in the atmosphere were doubled from pre-industrial levels. That 1979 study said the outcome range was "probable" -- less definitive than the language used today, and implying a roughly 50% chance that the actual temperature would fall in that range.

"The full range hasn't changed all that much in 35 years," Oppenheimer said. "And now, the most recent estimate is that 66% of the outcomes lie between 1.5 and 4.5 degrees."

That small improvement is hard won and represents significant advances in the science and data.